1 of 22 Markov Modeling of Fault-Tolerant Wireless Sensor Networks Arslan Munir and Ann Gordon-Ross + Department of Electrical and Computer Engineering University of Florida, Gainesville, Florida, USA This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) + Also affiliated with NSF Center for High-Performance Reconfigurable Computing

2 of 22 Wireless Sensor Networks (WSNs) WSN Applications Security and Defense Systems Health Care Ambient conditions Monitoring, e.g., forest fire detection Industrial Automation Logistics Ever Increasing

3 of 22 WSN Design Challenges WSN Design Challenges Meeting application requirements e.g., reliability, lifetime, throughput, delay (responsiveness), etc. Application requirements change over time Environmental conditions (stimuli) change over time Failure to meet Catastrophic Consequences Forest fire could spread uncontrollably in the case of a forest fire detection application Loss of life in the case of health care application Major disasters in the case of defense systems

4 of 22 WSN Hierarchical View Lifetime Reliability MTTF Application Requirements Gateway Sensor Nodes WSN Cluster WSN Cluster Head Sink Node Computer Network WSN Designer Sensor Field Average time to WSN failure

5 of 22 WSN Fault-Detection and Fault-Tolerance WSN Fault-detection & Fault-tolerance Fault-detection => Distributed fault-detection (DFD) algorithms that identify faulty sensor readings to indicate sensor faults Necessity Sensor nodes deployed in unattended and hostile environments => susceptible to failures Manual inspection of faulty sensor nodes after deployment is impractical Many WSN applications are mission critical and require continuous operation DFD algorithm’s accuracy => Algorithm’s ability to accurately identify faults Fault-tolerance => Adding hardware/software redundancy to the system

6 of 22 WSN Fault-Tolerance WSN Fault-tolerance Fault-tolerance => Adding hardware/software redundancy to the system Where to add redundancy? Within a sensor node Within a cluster => redundant sensor nodes Within a WSN => redundant WSN clusters Sensors (e.g., temperature and humidity sensors) have higher failure rates than other components (e.g., processors, transceivers, etc.) Sensors are cheap => Adding spare sensors contribute little to a sensor node’s cost

7 of 22 Contributions Investigate synergy between fault-detection and fault-tolerance (FT) for WSNs Fault-tolerance for WSNs Develop a Markov model for characterizing WSN reliability and MTTF Proposed a duplex sensor node model for fault-tolerance Helps WSN designer in determining the exact number of sensor nodes required to meet the application’s lifetime and reliability requirements Provides an insight into the type of sensor nodes (duplex or simplex) feasible for an application to meet the application’s requirements A good DFD algorithm is necessary for robust FT Used a hierarchical approach => WSN Markov model leverages WSN cluster model which in turn leverages sensor node model

8 of 22 NFT and FT Sensor Node Markov Model Non-fault-tolerant (NFT) Sensor Node Markov Model Fault-tolerant (FT) Sensor Node Markov Model = Sensor failure rate C = Coverage factor Probability that the faulty sensor is correctly diagnosed, disconnected, and replaced by a good inactive spare sensor Failed State 1 Non-Faulty Sensor 2 Non-Faulty Sensors Proposed Duplex Model

9 of 22 FT Sensor Node Markov Model – Differential Equation Solutions Reliability where – P i (t) = Probability that sensor node will be in state i at time t MTTF Average Failure Rate (or Failures in Time (FIT)) where – k denotes the average number of sensor node neighbors –k is important as DFD algorithm’s accuracy depends upon k

10 of 22 FT WSN Cluster Model k min +1n n-1 k min k min +1 k min k min +2 Average number of nodes in each cluster is n Average number of neighbor nodes per cluster is k = n - 1 We consider a special case with n = k min +2 A cluster fails to perform its assigned application task if the number of non-faulty sensor nodes in the cluster reduces to k min

11 of 22 FT WSN Cluster Markov Model – Differential Equation Solutions (n = k min +2) Reliability MTTF

12 of 22 FT WSN Cluster Markov Model – Differential Equation Solutions (n = k min +2) Average Failure Rate where – MTTF c(n) denotes the MTTF of a WSN cluster of n sensor nodes Sensor Nodes WSN Cluster WSN Cluster Head Sink Node Sensor Field

13 of 22 FT WSN Model N min +1N N-1 N min λ c(n) corresponds to the failure rate of a WSN cluster with n sensor nodes where –n s denotes the total number of sensor nodes in the WSN –n denotes the average number of nodes in a cluster A typical WSN consists of N = n s /n clusters WSN fails to perform its assigned task when the number of alive clusters reduces to N min

14 of 22 FT WSN Markov Model – Differential Equation Solutions (N = N min +2) Reliability MTTF

15 of 22 Experimental Results SHARPE Software Package –Markov model implementations NFT sensor node FT sensor node NFT WSN cluster FT WSN cluster NFT WSN FT WSN Sensor Failure Probability –Exponential distribution where –p = sensor failure probability –λ s = sensor failure rate over period t s –t s = time over which sensor failure probability/failure rate is specified –We present results for t s = 100 days Sensor Nodes WSN Cluster WSN Cluster Head Sink Node Sensor Field Our Markov models are valid for any other distribution as well

16 of 22 Results – MTTF FT & NFT Sensor Nodes MTTF (days) for an FT and a non-FT (NFT) sensor node. c = coverage factor k = number of neighbor sensor nodes c = 1 for an ideal DFD algorithm and an ideal replacement of faulty sensor with a non-faulty sensor An FT sensor node has a remarkable difference in MTTF as compared to an NFT sensor node: approx 95% improvement!! c improves as the number of neighboring sensor nodes k increases (DFD specifics) and hence the MTTF for an FT sensor node increases MTTF for both NFT and FT sensor node decreases as p increases

17 of 22 Results – Reliability FT & NFT Sensor Node Reliability for an NFT and an sensor node pNFTFT (k=5, c≠1)FT (k=10, c≠1)FT (c=1) The percentage improvement in reliability for an FT sensor node with c=1 over an NFT sensor node and FT sensor node with c≠1 and k = 5 and 10 is 230%, 172%, and 166%, respectively for p=0.9 The percentage improvement in reliability attained by an FT sensor node with c = 1 increases as p →1 because because for an FT sensor node with c≠1, c decreases as p →1 (DFD specifics) 230% 172% 166%

18 of 22 Results – MTTF FT & NFT WSN Cluster MTTF (days) for the FT and non-FT (NFT) WSN clusters with k min = 4. MTTF for FT WSN clusters with c=1 is always better than the FT WSN clusters with c≠1 The FT WSN cluster with n=k min +5 has more redundant senor nodes as compared to the FT WSN cluster with n=k min +2 and thus has a comparatively greater MTTF MTTF for both NFT and FT WSN cluster decreases as p increases

19 of 22 Results – Reliability FT & NFT WSN Cluster Reliability for an NFT and an FT WSN cluster when n = k min + 2 (k min = 4) pNFTFT (c≠1)FT (c=1) x x x x x10 -5 The percentage improvement in reliability for an FT WSN cluster with c=1 over an NFT WSN cluster and FT WSN cluster with c≠1 is 601% and 143%, respectively for p= % 143%

20 of 22 Results – MTTF FT & NFT WSN MTTF (days) for the FT and non-FT (NFT) WSNs with N min = 0. MTTF percentage improvement for FT WSN over NFT WSN is 88% for p=0.1 MTTF for WSNs with N=N min +5 is always greater than the MTTF for WSNs with N=N min +2 (due to additional redundancy in WSN with N=N min +5) MTTF percentage improvement for FT WSN over NFT WSN drops to 3.3% for p=0.99 Low failure probability sensors are required to build a more reliable FT WSN

21 of 22 Results – Reliability FT & NFT WSN Reliability for an NFT and FT WSN when N = N min + 2 (N min = 0) pNFTFT (c≠1)FT (c=1) An FT WSN can deliver much better reliability than an NFT WSN even for higher sensor failure probability p The percentage improvement in reliability for an FT WSN with c=1 over an NFT WSN and FT WSN with c≠1 is 350% and 236%, respectively for p= % 236%

22 of 22 Conclusions We proposed an FT duplex sensor node model –A novel concept for determining the coverage factor using sensor fault detection algorithm accuracy We developed hierarchical Markov models for WSNs consisting of sensor node clusters to compare the MTTF and reliability for FT and NFT WSNs –Aids design of WSNs with different lifetime and reliability requirements Fault detection algorithm’s accuracy plays a critical role in FT WSNs FT duplex sensor node provides improvement over an NFT sensor node –100% MTTF improvement with a perfect fault detection algorithm (c = 1) –MTTF improvement from 96% for current fault detection algorithms with low p (p < 0.3) - MTTF improvement reduces to 1.3% as p → 1 Percentage reliability improvement for an FT WSN with c = 1 over an NFT WSN with c≠1 is 350% and over an FT WSN with c≠1 is 236% for p=0.9 Redundancy in WSN plays an important role in improving MTTF –Our models allow designers to determine the fault detection algorithm’s accuracy and required redundancy to meet application requirements